Hypertherm MAX200 Service Manual - Manual (Page 45)

3-4 MAX200 Service Manual
MAINTENANCE
Theory of Operation: “Chopper” Power Supply
Introduction
The MAX200 plasma unit uses a “chopper” control circuit to stabilize the DC output current of the system. The
chopper is a switching circuit that is AC activated and outputs a square wave with the peak value being equal to its
DC input (283 VDC in the case of the MAX200).
Circuit Description
The following is a description of the operation of the chopper section of the MAX200 power supply. See wiring
diagram 013-4-179 as well as component figures in the Parts List section for reference.
“Snubber” Section
The “snubber” section supplies 283VDC to the chopper PCBs.
Incoming AC voltage enters dual snubber PCBs (PCB1, PCB3) and becomes rectified to DC through diodes D1-
D6. Varistors MOV1, MOV2, and MOV3 limit transient voltage spikes. Capacitors C1-C6 are charged to 283VDC.
Bleeder resistors R7 and R8 allow capacitors C1-C6 to discharge when the unit is not in use.
Chopper Section
Chopper modules PCB2 and PCB4 are powered by 14VAC from T3 and T4. Current control, which controls pulse
duration (duty cycle) of the choppers is selected from Voltage/Current (V/C) Remote or thumbwheel switch. This
information is on pins 5 and 6 of REC1 on the chopper PCBs. A longer pulse duration yields a higher output
current, and a shorter pulse duration yields a lower output current. The chopper module generates a square wave
of output current at the input DC voltage (283V) at an operating frequency of16KHZ.
The output of the chopper goes to the surge injection circuit (R4, C7), filter inductors (L1, L2), chopper module
diode, torch electrode, transferred arc sensor (CS1), and shunt resistors (R1, R2, R3). Fuses F3 and F4 protect
each chopper from exceeding 125 amps of output.
Surge Injection Circuit
Surge injection circuit capacitor C7 charges to the open circuit voltage of the chopper module (283VDC). When the
arc is transferred from the electrode to the workpiece (see THEORY OF OPERATION: HIGH FREQUENCY later in
this section for more information on arc transfer), C7 discharges through the 5
Ω
R4 and on to the torch electrode.
Current provided by C7 maintains the cutting arc, while the chopper module and rectifier circuit responds to the
current load of the cutting arc. Filter inductors (L1 and L2) and the chopper module diode begin to function once the
arc is established.